The present invention relates to navigation satellite receivers, and more particularly to methods and systems for accurately estimating the frequency of the local reference oscillator in a satellite navigation receiver during initialization.
The global positioning system (GPS) is a satellite-based radio-navigation system built and operated by the United States Department of Defense at a cost of over $13 billion. Twenty-four satellites circle the earth at an altitude of 20,200 km, and are spaced in orbit such that at any time a minimum constellation of six satellites is visible to any user. Each satellite transmits an accurate time and position signal referenced to an atomic clock. A typical GPS receiver locks on to this atomic clock, and then can very accurately measure the time delay for the signal to reach it, and the apparent receiver-satellite distance can then be calculated. Measurements from at least four satellites allow a GPS receiver to calculate its position, velocity, altitude, and time.
A GPS receiver that has just been turned-on does not yet know where it is, how much its crystal oscillator is in error, nor what time it is. All these are needed to find and lock onto the satellite transmissions, and so a search must be made of all the possibilities. The search has to be wider for large initial uncertainties in local time and reference clocks. The search effort can be reduced if the frequency and time can be tightly controlled or closely estimated.
The crystal oscillator in a GPS receiver that has not yet locked onto a satellite is basically free running. The typical phase locked loop circuit is not in lock. The actual operating frequency error can be quite large, and is highly dependent on temperature due to the nature of piezoelectric crystals. The temperature and other biases create a frequency uncertainty that increases the radio spectrum that must be searched during initialization. The time uncertainty further adds to the search problem during initialization because the satellite ID can be wrong or the expected Doppler shift can be wrong.
High-sensitivity GPS receivers create even more of a problem when the time or frequency uncertainty is large. Finding signal energy when that energy is extremely faint requires making smaller steps and dwelling at each step longer. So having a better initial estimate of the local reference oscillator can improve time-to-first-fix.
It is therefore an object of the present invention to provide a GPS receiver that can compensate its local crystal oscillator to reduce frequency uncertainties during initialization.
Briefly, a navigation-satellite receiver embodiment of the present invention comprises a crystal oscillator that is affected by local ambient temperature in a repeatable way. During lock on a GPS satellite, the receiver is used to calculate the true frequency bias of the local crystal oscillator. The current temperature of the crystal is measured and recorded in association with the true frequency bias measurement. The data is then used to generate a ninth-order polynomial that describes the frequency drift of the crystal over temperature. Then during receiver initialization when the local reference oscillator is not in lock, the ambient temperature is measured and used to index the ninth-order polynomial to estimate the actual crystal frequency. Such frequency estimate is then used as a basis to find signal from visible SV""s in an overhead constellation.
An advantage of the present invention is that a system and method are provided for faster initialization of navigation satellite receivers.
Another advantage of the present invention is that a system and method are provided for improving the sensitivity navigation satellite receivers.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.